1. Field of the Invention
The present invention relates to an apparatus for measuring a wave front aberration, a method for measuring a wave front aberration using the apparatus, and a method for manufacturing a projection lens by using thereof.
2. Description of Related Art
Recently, a projection lens of a projection exposure apparatus has been required to have higher resolving power in order to cope with the recent improvement of the density and the minuteness of a semiconductor device.
An optical performance of a projection lens has been evaluated by a wave front aberration because the wave front aberration is the sole measure capable of foreseeing resolution of any pattern.
There are several methods for measuring a wave front aberration. Among them, a method that measures a wave front aberration by a phase retrieve method based on point-spread functions of an object at a focused position and a defocused position and known information has been proposed in following references: J. Maeda at al. xe2x80x9cRetrieval of wave aberration from point-spread function or optical transfer datexe2x80x9d Applied Optics vol. 20, p274-279, D. L. Misell xe2x80x9cAn examination of an iterative method for solution of the phase problem in optics and electron opticsxe2x80x9d Test calculations Journal of physics D: Applied Physics vol. 6, p2200-2216.
However, when an apparatus for measuring a wave front aberration is realized based on the theory, since it is impossible to directly detect the point-spread function by a CCD on account of limited number of pixels of the CCD, it is inevitable for the apparatus f or measuring a wave front aberration to have an magnifying projection optical system for magnifying the point-spread function.
In this case, it is necessary to eliminate aberrations of the magnifying projection optical system. Thus the aberrations of the magnifying projection optical system have to be measured. This is fit for calibration. In the phase retrieve method from a point image, calibration method for the apparatus has not been found out.
Moreover, deformation of a point image may be caused not only by a wave front aberration of a test lens but also by an unnecessary element caused by a construction of the point image. When the deformation of a point image includes other elements than the wave front aberration of the test lens, the wave front aberration of the test lens cannot be measured correctly.
FIG. 10 is a drawing showing a construction of an ordinary point source 71. The point source 71 is constructed by a pinhole 74 arranged on a shield film 73 formed on a glass substrate 72, a lamp 75 for illuminating the pin hole 74 with a parallel light L11 from the shield film 73 side, and an illumination lens 76.
In the point source 71, a light L12 passed through the pinhole 74 is exit as a measuring light L13 after passing through the glass substrate 72. Then, the light L13 is converged by a test lens (not shown), and forms a deformed point image (an image of the pinhole 74).
In this case, a light L12 just passed through the pin hole 74 is approximately a spherical wave. However, a light incident to the test lens (not shown) is the measuring light L13 affected with aberrations (spherical aberration and coma) caused by the glass substrate 72 while passing through the glass substrate 72. Accordingly, the point image obtained by converging the measuring light L13 inevitably includes deformation in accordance with aberrations caused by the glass substrate 72.
Further, the parallel light L11 illuminating the pinhole 74 has an angular divergence xcex8 determined by the focal length of the illumination lens 76 and the aperture diameter d of the lamp 75. Since the angular divergence xcex8 appears directly on an angular divergence "psgr" of the measuring light L13 after passing through the glass substrate 72, the point image contains deformation in accordance with the angular divergence "psgr" (which is equal to the angular dispersion xcex8 of the parallel light L11) of the measuring light L13.
Furthermore, when a flare light L14 is incident to the pinhole 74, since an unnecessary light caused by the flare light 14 is mixed with the measuring light L13, deformation caused by the flare light 14 is added to the point image.
Thus when the wave front aberration of the test lens is measured by using the point source 71 having the construction shown in FIG. 10, deformation of the point image includes influence of unnecessary elements (aberrations in the glass substrate 72, an angular dispersion "psgr" of the measuring light L13, a flare light L14), so that the wave front aberration of the test lens cannot be measured precisely.
Moreover, although influence of aberrations of the glass substrate 72 can be corrected after the calculation using the phase retrieve method, it has not been easy because the thickness and the inclination of the glass substrate 72 have to be measured precisely.
Further, although the influence of the angular dispersion "psgr" of the measuring light L13 can be relieved by making the angular dispersion xcex8 of the parallel light L11 smaller by using smaller aperture diameter d, it is not desirable that the light quantities of the parallel light L11 as well as the measuring light L13 decrease in correspondence with decrease in the aperture diameter d.
The present invention is made in view of the above-mentioned problems and has an object to provide a method and apparatus for measuring a wave front aberration of a projection lens with high precision and a calibration method of the apparatus for measuring a wave front aberration.
According to an aspect of the present invention, an apparatus for measuring a wave front aberration includes: either a light source and an element that produces a first point source in combination with the light source and that is removably or movably arranged or a first point source generating part; a holding mechanism that holds a test object; an magnifying projection optical system that projects and enlarges a point image of the first point source projected by the test object; a detector that detects the magnified point image projected and magnified by the magnifying projection optical system; a supporting member that supports the magnifying projection optical system and the detector and that can be moved along the optical axis and in a plane perpendicular to the optical axis; a calculating part that calculates a wave front aberration by means of a phase retrieve algorithm based on a point-spread function detected by the detector and known information input in advance; and either a second point source producing element that is removably or movably arranged and that produces a second point source on the image plane of the test object by means of the test object in combination with any one of the light source, the light source and the element, and the first point source generating part, or a second point source generating part that produces the second point source on the image plane of the test object and that is removably or movably arranged.
The point image of the first point source is projected by the test object in the image plane of the test object.
The image is deformed by the aberration of the test object relative to an ideal point image.
A point image which is further projected and magnified image by the magnifying projection optical system is further deformed by an aberration of the magnifying projection optical system.
Accordingly, the point image formed on the CCD includes the aberration of the test object superimposed by the aberration of the magnifying projection optical system.
Therefore, in order to measure the wave front aberration of only the test object, it is necessary that the aberration of only the magnifying projection optical system is measured first, and, then, the aberration of the test object and that of the magnifying projection optical system is subtracted by that of only the magnifying projection optical system.
At first, the second point source is generated in the image plane of the test object and, then, the aberration of only the magnifying projection optical system can be calculated by the phase retrieve algorithm based on the point-spread function formed by the detector via the magnifying projection optical system and known information.
In other words, since an ideal light from a point source is projected on the detector via the magnifying projection optical system, the point-spread function of the image contains the aberration of only the magnifying projection optical system.
According to another aspect of the present invention, an apparatus for measuring a wave front aberration includes: a point source that has a shield member and an illuminating member for illuminating a pinhole part made on the shield member from one side, and that emanates a measuring light from the other side of the pinhole part; a holding mechanism that holds a test lens; and a detector that detects the intensity distribution of the point image on the other side projected by the test lens; wherein the diameter of the pinhole part made on the shield member of the point source, facing to the one side illuminated by the illuminating member is larger than that on the other side emanating the measuring light.